1. Field of the Invention
This invention relates to a method and apparatus for producing hydrogen and oxygen from water using photoelectrolysis. More particularly, this invention relates to a method and apparatus for photoelectrolysis of water to produce hydrogen and oxygen using sunlight illumination.
2. Description of Related Art
Photoelectrochemistry is the study of the interaction of light (in particular, radiation in the “sunlight” region, about 87 to 308 kJ/mole or about 0.9 to about 3.2 eV per photon) with electronic flow and chemical reactions at the electrode surfaces in electrochemical cells. The radiation involved in this process has considerable energy and can be used for the direct production of electricity and the splitting of water into hydrogen and oxygen, referred to as photoelectrolysis. However, to be practical, efficient and inexpensive systems utilizing readily available materials must be devised for the conversion process. The hindrances to practical applications of the system include the poor stability and low efficiency of the photoelectrode due to photoelectrochemical reactions involving photon-electron transfer and recombination, redox exchange and surface corrosion. For photoelectrolysis to be competitive with conventional fuel sources, it is necessary that these problems be addressed.
Present day photoelectrolysis systems employ a fairly thick electrolyte that limits the transmission of sunlight to and the departure of product gases from the photoelectrodes due to electrolyte surface tension. The current design of photo electrodes is an additional hindrance to the development of improved photoelectrochemical systems because the semiconductors employed therein are fabricated on conductive substrates. With this type of design, there is no way to reduce the thickness of the electrolyte layer and eliminate the surface tension that acts as an inhibitor to the release of product gases because the reactant water and electrolyte must be transported to the front of the electrode.
Numerous efforts have been made to enhance the efficiency and stability of photoelectrochemical cells. The general approach has been to coat a layer of protective materials, which may be organic substances, active metal ions, noble metals, light sensitive dyes and more stable semiconductors, such as metal oxides, onto the photoelectrode surface. Recent developments include a thin film dye to sensitize the semiconductor electrodes in photoelectrochemical cells. Although the use of light sensitive dyes on the semiconductor electrode surface has improved the light absorption efficiency thereof, it is still necessary that the mass transport rate be increased and that the electrolyte thickness be reduced.